Mary O'Brien may refer to:
- Mary Ann O'Brien (born 1960), Irish politician
- Mary Eileen O'Brien, American academic administrator
- Mary F. O'Brien, United States Air Force general
- Mary Grace O'Brien (born 1958), judge of the Virginia Court of Appeals
- Dusty Springfield (Mary Isobel Catherine Bernadette O'Brien, 1939âÂÂ1999), British pop singer
- Mary K. O'Brien (born 1965), American politician and judge
- Mary O'Brien, 3rd Countess of Orkney (1721âÂÂ1791), mute Irish countess
- Mary O'Brien, 4th Countess of Orkney (1755âÂÂ1831), Scottish peeress
- Mary O'Brien (philosopher) (1926âÂÂ1998), feminist political philosopher
- Mary O'Brien (writer) (fl. 1785âÂÂ1790), Irish poet and playwright
- Mary O'Brien Harris (1865âÂÂ1938), British politician